Problems with update manager in Ubuntu 11.10

IN THE UPDATE MANAGER OF UBUNTU 11.10, I JUST CAN'T MANAGE TO DOWNLOAD REPOSITORY INFORMATION. THE DETAILS OF MY PC OUTPUT ARE: "W:Failed to fetch cdrom://Ubuntu 11.10 _Oneiric Ocelot_ - Release i386 (20111012)/dists/oneiric/main/binary-i386/Packages Please use apt-cdrom to make this CD-ROM recognized by APT. apt-get update cannot be used to add new CD-ROMs
, W:Failed to fetch cdrom://Ubuntu 11.10 _Oneiric Ocelot_ - Release i386 (20111012)/dists/oneiric/restricted/binary-i386/Packages Please use apt-cdrom to make this CD-ROM recognized by APT. apt-get update cannot be used to add new CD-ROMs
, E:Some index files failed to download. They have been ignored, or old ones used instead.".

CAN SOMEONE HELP ME TO SOLVE THIS PROBLEM, OR EXPLAIN TO ME WHAT PATH OF ACTION SHOULD I TAKE IN ORDER TO MAKE THE UPDATE MANAGER WORK PROPERLY???? THANK YOU VERY MUCH INDEED!!!!!!!!

Dear Andrew, thank you very much for your suggestions. The update manager seems to flow perfectly now, and it seems to run without any problmes. I just have one more doubt for which I am asking for your help........In the terminal, I have followed all your instructions, but in the end, when I run the command "sudo apt-get update", eventually, I find the following lines: "W: Duplicate sources.list entry http://archive.canonical.com/ubuntu/ oneiric/partner i386 Packages (/var/lib/apt/lists/archive.canonical.com_ubuntu_dists_oneiric_partner_binary-i386_Packages)W: we suggest you to run "apt-get update" in order to correct these problems. Can you explain why there is this strange information?????? Thank you!